A "Raman accumulator cell" is a closed container containing a Raman-active gas (e.g., Ch.sub.4, H.sub.2 or CS.sub.2) and having two parallel side walls that are substantially fully reflective (viewed from within the container) at two predetermined, adjacent frequencies .nu..sub.p and .nu..sub.s (.ltorsim..nu..sub.p) and having two opposing end walls that are substantially transparent at the frequencies .nu..sub.p and .nu..sub.s and at a third predetermined adjacent frequency .nu..sub.2s (.ltorsim..nu..sub.s).
Propagation of substantially monochromatic radiation of frequency, say .nu.=.nu..sub.p, in a Raman-active gas gives rise to stimulated Raman scattering at a series of frequencies .nu.=.nu..sub.p .+-..nu..sub.1, .nu..sub.p .+-..nu..sub.2 . . . (0 &lt;.nu..sub.1 &lt;.nu..sub.2 . . .), called Stokes frequencies, where the frequency shift {.DELTA..nu.{=.nu..sub.1, .nu..sub.2 . . . are usually small fractions of the central (Rayleigh) frequency .nu.=.nu..sub.p and manifest the effects of molecular vibrations of the Raman-active gas. Hydrogen gas (H.sub.2, HD and D.sub.2) and other low atomic weight molecular gases such as CH.sub.4, CS.sub.2, etc. have large vibrational shifts, but gases also having narrow line widths at the shifted frequencies are preferred for Raman scattering. The Raman scattered radiation can be a Stokes line (.DELTA..nu.=-.nu..sub.1, =.nu..sub.2 . . .) or an anti-Stokes line (.DELTA..nu.=.+-..nu..sub.1, .+-..nu. .sub.2 . . .) depending upon whether the particular molecular vibration of the gas absorbs energy (.DELTA..nu.&lt;0) or gives up energy (.DELTA..nu.&gt;0) as the gas scatters the incident radiation.
Raman cell conversion of energy (from one frequency to another) is a useful means of generating high intensity laser radiation, since radiation energy can be temporarily stored in the medium through Raman pumping and extracted by a Stokes wave of shifted frequency for pump power density above a predetermined threshold.
One problem encountered here is the appearance of higher order Stokes waves of different frequencies that often act as parasitic waves, growing in intensity at the expense of the first Stokes wave.